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Characterisation of Shigella Spa33 and Thermotoga FliM/N reveals a new model for C-ring assembly in T3SS.

Mcdowell, M.A. and Marcoux, J. and Mcvicker, G. and Johnson, S. and Fong, Y.H. and Stevens, R. and Bowman, L.A.H. and Degiacomi, M.T. and Yan, J. and Wise, A. and Friede, M.E. and Benesch, J.L.P. and Deane, J.E. and Tang, C.M. and Robinson, C.V. and Lea, S.M. (2016) 'Characterisation of Shigella Spa33 and Thermotoga FliM/N reveals a new model for C-ring assembly in T3SS.', Molecular microbiology., 99 (4). pp. 749-766.


Flagellar type III secretion systems (T3SS) contain an essential cytoplasmic-ring (C-ring) largely composed of two proteins FliM and FliN, whereas an analogous substructure for the closely related non-flagellar (NF) T3SS has not been observed in situ. We show that the spa33 gene encoding the putative NF-T3SS C-ring component in Shigella flexneri is alternatively translated to produce both full-length (Spa33-FL) and a short variant (Spa33-C), with both required for secretion. They associate in a 1:2 complex (Spa33-FL/C2) that further oligomerises into elongated arrays in vitro. The structure of Spa33-C2 and identification of an unexpected intramolecular pseudodimer in Spa33-FL reveal a molecular model for their higher order assembly within NF-T3SS. Spa33-FL and Spa33-C are identified as functional counterparts of a FliM–FliN fusion and free FliN respectively. Furthermore, we show that Thermotoga maritima FliM and FliN form a 1:3 complex structurally equivalent to Spa33-FL/C2, allowing us to propose a unified model for C-ring assembly by NF-T3SS and flagellar-T3SS.

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Publisher statement:© 2015 The Authors. Molecular Microbiology published by John Wiley & Sons Ltd. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Date accepted:02 November 2015
Date deposited:07 August 2017
Date of first online publication:23 December 2015
Date first made open access:No date available

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